The present invention relates to the general field of methods of fabricating parts by casting. The invention relates more particularly to a mold for fabricating a monocrystalline turbine engine blade by lost wax casting.
In certain circumstances, and in particular in aviation turbine engines, it is necessary to have metal or metal alloy parts that present a controlled monocrystalline structure. For example, in turbine nozzles of aviation turbine engines, the blades need to withstand large thermomechanical stresses due to the high temperature and to the centrifugal forces to which they are subjected. A controlled monocrystalline structure in the metal alloys forming these blades enables the effects of such stresses to be limited.
In order to make a metal part of this type, lost wax type casting methods are known. In known manner, in such a method, a wax model is initially made of the part that is to be fabricated, and a ceramic shell is then formed around it to form a mold. Thereafter molten metal is cast into the mold and directional solidification of the metal makes it possible, after removing the mold, to obtain the molded part or “casting”. This method is advantageous for fabricating metal parts of complex shapes, and it enables parts to be obtained that are of monocrystalline structure, e.g. by using a seed or a grain selector duct.
Blades generally comprise a root, an inner platform having lips, an airfoil, an outer platform having lips, and wipers. During the fabrication of monocrystalline blades by a method of the kind described above, certain problems appear, due in particular to the shape of the blades.
During the directional solidification of the molten metal present in the mold having the shape of the blade, those portions of the mold cavity that form in particular the lips of the inner and outer platforms solidify somewhat later than the other portions of the cavity such as those forming the airfoil. This delay can lead to undesirable pores appearing in the final part.
In addition, at the end of heat treatment that is performed after directional solidification, it has been observed that the blade may present interfering recrystallized grains at certain locations, and in particular on the leading edge or the trailing edge in the proximity of the inner or outer platforms. This is not desirable when it is desired to obtain a monocrystalline blade.
Finally, the blades that are obtained may present significant variation in dimensions between the wax model and the final part, and can sometimes be deformed or twisted.
There therefore exists a need to have a mold for fabricating a turbine engine blade and a method of fabricating such a blade that reduce the appearance of the above-mentioned defects.